Abstract: Solid alkaline silicates are continuously manufactured by the use of a rotary furnace including a tubular body rotating slowly with a slight rake, having an opening for charging the materials at its higher end, a burner at its lower end and several rectangular or squared holes for discharging the product in the lower end of its side wall. In the discharging holes are exteriorly fitted several interchangeable refractory bushings having different passage sections to regulate the product discharging rate according to the viscosity thereof. Preferably, the bushings are made up of two halves and means are provided on the outside of the furnace wall for retaining and positioning the bushings.